This invention relates to an automatic adjuster for use in a vehicle brake actuator and including a tappet for engagement, in use, with a brake shoe, an adjuster member forming a non-reversible threaded connection such that rotation of the member causes axial movement of the tappet, and adjusting mechanism for rotating the adjuster member to adjust the axial position of the tappet when required, said mechanism including a clutch ring carrying a reversible screw thread and co-operating with a corresponding reversible thread on the adjuster member or on means operatively associated with the latter. An adjuster of this general type is illustrated in FIG. 4 of our earlier British Pat. No. 1446552.
In shoe drum brakes arranged in a two leading shoe configuration, it is known to locate oppositely acting separate adjusters between the shoes in such a manner that for one direction of rotation, one of the adjusters adjusts one shoe and the other acts as an abutment for the other shoe, and vice-versa. Sometimes in an arrangement of this kind, a spring is placed between operative parts of the adjusters in order to cause the appropriate one of these parts to follow the leading shoe. Such a spring generates a force in the non-reversible thread which has to be overcome before adjustment can occur. As a result of this, torsional forces generated at the clutch ring can be unduly high, leading to rapid wear of the clutch faces. Moreover, when the adjuster is acting in the abutment mode, it has to sustain input force from the other leading shoe and a part of such force is transmitted through the clutch ring, further contributing to wear and possible distortion of the ring, and/or resulting in failure of the adjuster to operate with subsequent applications of the brake.
In addition to the above disadvantages, the force of the aforesaid spring provides too great a resistance to rotation of the adjuster components when manual de-adjustment of the device is required for servicing purposes.